1. Field of the Invention
The invention relates to over-current protection of a transformer supplying one or more loads and, in particular, to over-current protection of a transformer supplying power to model electric trains.
2. Description of the Related Art
Conventionally, transformers are used to adapt the available electrical supply from a generator, power supply or the common alternating current wall outlet to the voltage, current and power levels required by an electrical apparatus. A fuse is generally located in line with the primary of the transformer. When the fuse reaches a certain current level, it opens, protecting the transformer from overloads that may damage it permanently, such as overheating of the insulation. In many applications opening, or blowing, the fuse is undesirable. In these applications, the simple protection afforded by such a fuse has been supplemented by electronic protection.
One such application is in the operation of model toy trains. In this consumer electronics application, a blown fuse disables the use of the train until the fuse is replaced. Such fuses are generally not easily accessible or replaceable by the consumer. Electronic controls designed to minimize the current through the transformer to levels below the operation of the fuse protect the transformer from overloads without operation of the fuse. These controls can additionally protect other internal devices from excessive heat and power and provide a more enjoyable experience for the consumer.
Accordingly, an accurate apparatus and method of determining the load through a transformer is needed that provides appropriate over-current protection without appreciable reduction in the performance in the loads connected to the transformer. An inventive apparatus and method for more accurately performing phase control of the loads is also desirable, whether in combination with over-current protection or not.
Thus, the present invention includes a method for over-current protection of a transformer supplying an alternating current supply signal to a load. The method includes periodically sampling a current supplied to the load by the supply signal during a cycle of the supply signal, calculating a root-mean-squared (RMS) average current using the samples collected during the cycle, comparing the RMS average current to a target current for the cycle, and limiting an amount of power intended for the load during a subsequent cycle of the supply signal to the lower of a desired power value and an RMS average power value, the RMS average power value determined by the comparison of the RMS average current to the target current.
The invention also includes a method for selectively enabling flow of current to a load from an alternating current supply signal connected to the load. This method includes storing a first time passing between a first zero-crossing of the supply signal, the first zero-crossing indicating a leading edge of the supply signal, and a second zero-crossing of the supply signal, the second zero-crossing indicating a trailing edge of the supply signal, and sending a first signal enabling flow of the current according to a desired conduction angle when a subsequent zero-crossing indicating the leading edge of the supply signal does not occur within a second time, the second time being at least as long as the first time.
In a train controller for a model toy train wherein the train controller includes means for selectively enabling flow of an alternating current from a supply signal to a train track, an improvement of the present invention includes means for storing a first time passing between a first zero-crossing of the supply signal and a second zero-crossing of the supply signal, the first zero-crossing indicating a leading edge of the supply signal, and the second zero-crossing indicating a trailing edge of the supply signal; and a first signal enabling flow of the alternating current through the device according to a desired conduction angle when a subsequent zero-crossing indicating the leading edge of the supply signal does not occur within a second time, the second time being at least as long as the first time.
The invention also includes an apparatus including a controller capable of performing a process for over-current protection of a transformer supplying an alternating current supply signal to a load. The process includes periodically sampling a current supplied to the load by the supply signal during a cycle of the supply signal, calculating an RMS average current using the samples collected during the cycle, comparing the RMS average current to a target current for the cycle and limiting an amount of power intended for the load during a subsequent cycle of the supply signal to the lower of a desired power value and an RMS average power value, the RMS average power value determined by the comparison of the RMS average current to the target current for over-current protection of a transformer supplying an alternating current supply signal to a load.
Other applications and details of the present invention will become apparent to those skilled in the art when the following description of the best mode contemplated for practicing the invention is read in conjunction with the accompanying drawings.